COUNTERROTATING GASEOUS DISKS IN NGC-4826

Observations are presented of the system of nested counterrotating gaseous disks discovered in NGC 4826. Imaging spectroscopy in neutral hydrogen has been done using the VLA and the WSRT. Broad-band optical images in B, V, and I as well as narrow-band images in H alpha and [S II] have been obtained at the KPNO. Partial imaging along two position angles in the CO (3-->2) transition was carried out at the JCMT. All components for which kinematic data exist (neutral, molecular and ionized gas as well as a stellar component) within a radius of 1 kpc share the same sense of rotation. The inner gas disk has an extremely high gas surface density (greater than 50 M(.) pc(-2)) and high star formation rate. This inner disk is bounded by an annular region of low gas surface density with faint, diffuse Ha emission detected between 1 and 2.8 kpc radius. An outer gas disk is detected in neutral hydrogen extending from 1.5 to 11 kpc radius at a surface density of similar to 0.5 M(.) pc(-2). The H I kinematics indicate counterrotation of this component relative to the inner galaxy at an identical kinematic position angle and inclination. A luminous stellar disk dominates the optical continuum emission and extends out to a radius of 5.5 kpc with substantial surface brightness. Recent optical spectroscopy has shown that the outer stellar disk shares the kinematics of the inner galaxy. It seems likely that the stellar disk decelerates the tenuous, co-planar, counterrotating outer gas disk through a strong interaction with its stellar mass loss. The observed system of nested, counterrotating gaseous disks may have arisen in the antiparallel spin merger of a gas poor spiral with a star-poor dwarf, each containing a few times 10(8) M(.) of gas. An alternate formation scenario calls for the continuous accretion of a few times 10(8) M(.) of antiparallel spin gas. The interaction of gaseous disks with stellar mass loss may also play an important role in the evolution of polar ring galaxies as well as more generally in bulge-dominated galaxies.